Hi Markus,
I would be very suspicious of that request.
Once you have signed your lease (which it sounds like you did years ago when you first moved in) your landlord cannot compel you to sign a new agreement, unless you are deliberately changing the details of your lease (like, if you are adding a new parking space, or storage locker, or something like that). And in my experience, when a landlord wants new forms to be filled-out, then they are up to something.
I mean, in your case this all could be an innocent information-gathering activity.
But do you have to sign those forms? Are the forms asking for sensitive information (like your place of employment, salary, banking info)? Do the forms say anything about giving the landlord the right to run additional credit-checks? I would be very leery of providing that information, since you already have the apartment.
Lastly, depending upon the exact wording on the forms, could those rental application forms be construed to constitute a new agreement with the landlord, thus allowing them to change the terms of your leases, or to set a new monthly rent?
Be careful!